Nd—Fe—B sintered magnets have been widely used in various fields such as electronics and information technology, automobiles, medical equipment, energy, and transportation, etc. Meanwhile, with the continuing improvement of technology and reduction of cost, Nd—Fe—B permanent magnets find wide potential applications in many emerging fields. With the advent of low-carbon economics, countries have paid attention to environmental protection and low carbon emissions as key science and technology fields. Therefore energy structure improvement, renewable energy development, increased energy efficiency, reduced energy consumption and carbon emission are in demand. New market emerges in low carbon industries such as wind-power generators, new-energy vehicles, energy-saving home appliances, etc. The new applications require improved performance of Nd—Fe—B sintered magnets. For example, the most popular laptop computers are equipped with 2.5-inch hard disks. The voice coil motors (VCM) of hard disks require N50H-grade Nd—Fe—B sintered magnets with the maximum energy product (BH)max>48 MGOe and intrinsic coercivity Hcj>16 kOe. In another example, the thin plate high performance Nd—Fe—B magnets in ignition coil of automobile engines operate at a required working temperature higher than 200° C. the application requires N35EHS-grade sintered Nd—Fe—B magnets with (BH)max>33 MGOe and coercivity Hcj>35 kOe. Both high (BH)max and high Hcj are demanded of Nd—Fe—B magnets in emerging applications such as robotic walkers, integrated special motors, and automatic driving systems, etc. Rare earths are important strategic resources. Enhanced comprehensive magnetic properties of Nd—Fe—B sintered magnet improve efficient use of these resources. Therefore, the trend of developing Nd—Fe—B sintered magnets is to improve both (BH)max and Hcj simultaneously.
Currently the major global manufacturers have launched high performance Nd—Fe—B sintered magnet products to meet specific purpose requirements. Hitachi Metals Co. has developed Nd—Fe—B sintered magnets with (BH)max of 53 MGOe for stable production; Vacuumschmelze (VAC) in Germany has put magnets of 50 MGOe (BH)max into mass production, and TDK Co. in Japan has also put commercial magnets with 48˜50 MGOe (BH)max into mass production. However, none of the products achieves both high (BH)max and high Hcj. The typical magnetic properties of some of the commercialized high performance magnets are listed in Table 1.
TABLE 1Magnetic properties of Nd—Fe—B sintered magnets withhigh performance produced by some global manufacturersHcj/(BH)max/GradeCompanyBr/kGskOeMGOeNMX-S54Hitachi Metals Co.14.5~15.11151~55NMX-S41EHHitachi Metals Co.12.4~13.12537~42NMX-S34GHHitachi Metals Co.11.2~12 3330~35VACODYM688TPVAC11.43632VACODYM745HRVAC14.41547
Table 1 shows that Nd—Fe—B sintered magnets with high (BH)max correlate to low Hcj. Similarly, the high Hcj correlate to relatively low (BH)max. In addition, the numeric sum of (BH)max and Hcj of all products fall between 60 and 70.
The fundamental function of permanent magnets is to provide magnetic fields in application spaces. The maximum energy product (BH)max (MGOe) represents the capacity of a permanent magnet to provide magnetic energy output. With the same size, a permanent magnet of larger (BI)max provides stronger magnetic field. The intrinsic coercivity Hcj (kOe) represents the capability of a magnet to keep itself stable in magnetized state. If Hcj of a magnet is not high enough, Hcj decays when the magnet is disturbed by demagnetizing field, temperature, or vibration, whereby the capacity of part or the whole of the magnet to provide magnetic field decreases, i.e., the capability of the magnet to maintain its magnetized state and to supply the magnetic field eventually decreases.
For Nd—Fe—B sintered magnets, the relationship between Hcj and (BH)max or Remanence Br tends to be antagonistic. The magnet with high Hcj has decreased (BH)max or Br. Hcj decreases if (BH)max or Br is enhanced. It follows that unconditional increase of the Hcj would significantly affect (BH)max and decrease parameters and comprehensive characteristics of the magnet, and limit the applicability of the magnet. Therefore, in the field of Nd—Fe—B sintered magnets, the sum of (BH)max and Hcj is considered to be a comprehensive parameter for the performance of a magnet.